Ion channels and neurotransmitter receptors are amongst the most important molecules in the nervous system. They support the high-speed physiological processes that enable neurons to function, they are implicated in many neurological and psychiatric disorders, and they present an incredibly important set of drug targets for treating these diseases. In order to enable a better understanding of how specific ion channels and receptors contribute to behaviors and pathologies, we propose to engineer a toolbox of fully genetically encoded reagents that, when expressed in specific neurons in the brain, enable specific ion channels and ionotropic neurotransmitter receptors to be driven or blocked in a temporally precise fashion, using pulses of light. We anticipate that these tools will find widespread use in both basic and clinical neuroscience, and in other fields of biology, for revealing the roles that specific ion channels and receptors (or changes in their activity levels) play in neural computations, behaviors, and disease states, and for revealing more specific drug targets.